1. Field of the Invention
The present invention relates to a method and device for data commmunication usable for communication of data between terminals, computers or the like installed as an in-house distributed processing system.
2. Description of the Prior Art
Owing to the spread of in-house distributed processing, a data communication method has been widely employed in which a large number of terminals and computers are installed as an in-house distributed processing system and communication is carried out among them.
The method is such that a coaxial cable is employed as a transmission line and the signals sent through the coaxial cable are led to a terminal or a computer provided in each of the rooms through a transceiver, signal line and connector provided for each room, thereby carrying out data communication.
In this case, there are provided a carrier sense line, contention detect line, data send line and data receive line, as the interface between the transceivers and the terminals or the computers (referred to as "data processing equipment" hereinafter). The carrier sense line is in the "1" state when another transceiver is sending data on the coaxial cable, but is in the "0" state in other cases. The contention detect line is in the "1" state when another transceiver attempts to send data at the same time when the data processing equipment is sending data, but is in the "0" state in other cases. The data send line is used when the data processing equipment sends data. On the other hand, the data receive line is used when the data processing equipment receives data.
Employing this interface, the conventional method allows data to be transferred as follows.
Any one of the data processing equipment can start the sending of data through the data send line when the carrier sense line is in the "0" state. The data is sent to the coaxial cable by means of the corresponding transceiver. The data is transferred to the coaxial cable with a propagation delay and received by another transceiver, which brings the carrier sense line into the "1" state for showing that the coaxial cable is being used. However, owing to the propagation delay, when the carrier sense line of the second-mentioned transceiver is in the "0" state even after the first-mentioned transceiver has started data sending, there are cases where the data processing equipment connected to the second-mentioned transceiver starts data sending. In such cases, the first-mentioned transceiver detects the contention and brings the contention detect line into the "1" state. When the contention detect line is brought into the "1" state, the data processing equipment connected to the first-mentioned transceiver suspends the data sending and starts sending of the same data on confirmation that the carrier sense line is in the "0" state after the elapse of a time determined according to random numbers.
In such a system, when the carrier sense line is judged to be in the "0" state, i.e., when the coaxial cable is judged to be not used, data is immediately sent. Therefore, in case of occurrence of contention, the data sending must be stopped halfway and moreover, there is a need for such a troublesome processing that the data must be sent once more from the top thereof.
Furthermore, since transmission is once closed when contention occurs, it is not always possible for the data processing equipment having been made to close transmission, to resume data sending with priority to the others after the contention is removed, and there may be cases where another data processing equipment which attempts to start transmission starts data sending prior to the data processing equipment having been suspended because of contention.